Loading... # 多态 公有继承派生类对象使用基类的方法,如果希望同一种方法派生类和基类的行为是不同的,也即方法的行为取决于调用该方法的对象,这种行为称为多态。 * 基类使用虚方法(此时派生类自动成为虚方法,可以指出也可以不指出)。 * 基类不使用虚方法,在派生类直接重定义该方法。 ## 声明为virtual 如果不将函数声明为virtual,程序将根据引用类型或者指针类型选择方法。如果声明为virtual,程序将根据引用或者指针指向对象本身的类型来选择方法。 举例来说 ### 基类中方法和析构函数均不声明为virtual的情况 ```c #include <iostream> #include <memory> using namespace std; class Base { public: Base() { cout << "created Base\n"; } void function() { cout << "this is Base's function" << endl; } ~Base(){cout << "this is Base's destroy" << endl;} }; class Derived : public Base { public: Derived(){ cout << "created derived" << endl;} void function(){cout << "this is Derived's function" << endl;} ~Derived(){cout << "this is Derived's destroy" << endl;} }; int main() { Base* ptr = new Derived; // std::unique_ptr<Base> ptr(new Derived); ptr->function(); delete ptr; // delete 语句的根本含义在于调用其析构函数,并释放内存 return 0; } ``` 输出 ``` created Base created derived 可以看出构造函数不受影响,被执行了 this is Base's function 虽然对象是派生类,但是由于指针是指向基类的,所以执行了基类的方法 this is Base's destroy 虽然对象是派生类,但由于指针是指向基类的,所以只执行了基类的析构函数,所以该对象未被完全释放。 ``` ### 基类中方法与析构函数均声明为virtual的情况 ```c #include <iostream> #include <memory> using namespace std; class Base { public: Base() { cout << "created Base\n"; } virtual void function() { cout << "this is Base's function" << endl; } virtual ~Base(){cout << "this is Base's destroy" << endl;} }; class Derived : public Base { public: Derived(){ cout << "created derived" << endl;} void function(){cout << "this is Derived's function" << endl;} ~Derived(){cout << "this is Derived's destroy" << endl;} }; int main() { Base* ptr = new Derived; // std::unique_ptr<Base> ptr(new Derived); ptr->function(); delete ptr; // delete 语句的根本含义在于调用其析构函数,并释放内存 return 0; } ``` 输出 ``` created Base created derived this is Derived's function 执行的是派生类的方法,虽然指针指向的是基类 this is Derived's destroy 析构函数也被正确调用了, this is Base's destroy ``` ### 其他说明 若方法是通过对象(而不是指针或者引用调用的),则没有虚方法特性! 最后修改:2020 年 04 月 13 日 03 : 23 PM © 允许规范转载